constructor(public platformStrategy: LocationStrategy) {
    super(1);

    platformStrategy.onPopState(event => this._update('pop'));

    const browserBaseHref = this.platformStrategy.getBaseHref();
    this._baseHref = stripTrailingSlash(stripIndexHtml(browserBaseHref));
    this._update('push');
  }

 /**
  * Given a string representing a URL, returns the platform-specific external URL path.
  * If the given URL doesn't begin with a leading slash (`'/'`), this method adds one
  * before normalizing. This method will also add a hash if `HashLocationStrategy` is
  * used, or the `APP_BASE_HREF` if the `PathLocationStrategy` is in use.
  */
 prepareExternalUrl(url: string, query: any = ''): string {
   if (url.length > 0 && !url.startsWith('/')) {
     url = '/' + url;
   }
   return this.platformStrategy.prepareExternalUrl(url + normalizeQueryParams(normalizeQuery(query)));
 }

 back(): void {
     this.locationStrategy.back();
 }

 /**
  * Navigates forward in the platform's history.
  */
 forward(): void {
   this.platformStrategy.forward();
 }

 /**
  * Changes the browsers URL to the normalized version of the given URL, and replaces
  * the top item on the platform's history stack.
  */
 replace(path: string, query: any = ''): void {
   this.platformStrategy.replaceState(null, '', path, normalizeQuery(query));
   this._update('replace');
 }

 /**
  * Changes the browsers URL to the normalized version of the given URL, and pushes a
  * new item onto the platform's history.
  */
 go(path: string, query: any = ''): void {
   this.platformStrategy.pushState(null, '', path, normalizeQuery(query));
   this._update('push');
 }

 /**
  * Returns the normalized URL path.
  */
 path(): string {
   return this.normalize(this.platformStrategy.path());
 }

 /**
  * Navigates back in the platform's history.
  */
 back(): void {
   this.platformStrategy.back();
 }